Centrifugal compressor and vaned diffuser

Abstract: A centrifugal compressor comprises a centrifugal impeller (2) and a vaned diffuser (7). The diffuser (7) includes a pair of diffuser plates (3,4) and a plurality of guide vanes (5,6) arranged between the pair of diffuser plates (3,4) in a circular cascade manner. The guide vanes (5,6) are equal in leading-edge radius to each other, but are different in length from each other. At least one short guide vane (6) is arranged between the adjacent long guide vanes (5). The total number of the guide vanes (5,6) is more than the number of blades (13) of the impeller (2). A throat (10) is formed in only a part of flow passages between the guide vanes (5,6).

Description: The invention relates to a centrifugal compressor comprising a centrifugal impeller and a vaned diffuser arrangend downstream of said impeller, said vaned diffuser including a pair of diffuser plates and a plurality of guide vanes arranged between said pair of diffuser plates in a circular cascade manner, said circular cascade of said vaned diffuser comprising an inner circular cascade and an outer circular cascade, wherein the leading-edge radius of the guide vanes of the outer circular cascade is substantially the same in level as the trailing-edge radius of the guide vanes of the inner circular cascade. In centrifugal compressors, conventionally a vaned diffuser has been often used when high efficiency is required. These are disclosed in Japanese Utility Model Examined Publication No. 63-45599 and the like, for example. In such centrifugal compressor having the vaned diffuser, flow of fluid is guided by auide vanes so that flow angle measured from a tangential direction increases, and the flow of fluid is turned into a radial direction. Thus, the flow is efficiently decelerated. In this manner, performance of the centrifugal compressor having the vaned diffuser generally increases in efficiency, as compared with a case of a vaneless diffuser. In the vaned diffuser, however, throats are generally formed or defined respectively in passages between the adjoining guide vanes. Accordingly, choke occurs at large flow rate, and deceleration from a diffuser inlet to the throat increases at low flow rate so that the flow stalls and surge occurs. Thus, the prior art has a problem that operating range of compressor having the vaned diffuser is less than that of compressor having the vaneless diffuser. Further, flow of the fluid at an impeller outlet of the centrifugal compressor is non-uniform or uneven in a peripheral direction, that is, in a vane pitch direction. Accordingly, periodically fluctuating flow enters into the diffuser. Frequency of the fluctuating flow is equal to blade passage frequency of the impeller, that is, (the number of blades of the impeller) x (rotational frequency). Accordingly, since the fluctuating flow impinges against the guide vanes of the diffuser, compressor with the vaned diffuser has a problem that high noise which prevails by a blade passage frequency component is generated as compared with compressor with the vaneless diffuser. Moreover, in order to solve the problem that the operating range of compressor having the vaned diffuser is narrow, a diffuser which is low in solidity (vane length/cascade average pitch) has been invented and disclosed in Japanese Patent Unexamined Publication No. 53-119411. In the vaned diffuser, throats are not formed in passages between blades. Accordingly, choke and surge are difficult to be generated. Thus, the operating range of compressor with low solidity vaned diffuser is wide in the same degree or extent as that of compressor with the vaneless diffuser and is high in performance, but a problem that noise is high has not been solved. Furthermore, as disclosed in Japanese Utility Model Unexamined Publication No. 63-9500, a diffuser has also been proposed in which long guide vanes and short guide vanes are alternately arranged, and radii of tailing edges of all of the guide vanes are made constant. Further, as disclosed in U.S. Patent No. 4,824,325, a vaned diffuser has also been proposed in which guide vanes having low solidity are provided in two rows, a space in a radial direction is provided between a first row of guide vanes and a second row of guide vanes, and the number of guide vanes of the first row is twice the number of guide vanes of the second row. Centrifugal compressors having such diffusers are characterized to have wide operating range as compared with a centrifugal compressor having a conventional vaned diffuser. However, a problem that noise is high has not been solved. DE-A-38 35 622 shows a centrifugal compressor of the generic kind comprising a vaned diffuser which comprises an inner circular cascade and an outer circular cascade and in which a leading-edge radius of the guide vanes of the outer circular cascade is substantially the same in level as a trailing-edge radius of the guide vanes of the inner circular cascade and in which the number of guide vanes of the inner circular cascade is equal to the number of guide vanes of the outer circular cascade in order to suppress the growth of the boundary layer on negative pressure sides of the guide vanes of the outer circular cascade by blowing out the high velocity flow between the inner and outer circular cascades to increase the performance of the compressor. JP-A-61 093 299 shows a vaned diffuser in which a plurality of micro-vanes are provided between vanes. There are no inner and outer circular cascades. CH-A-317 623 shows a vaned diffuser which comprises an inner circular cascade and an outer circular cascade. The number of guide vanes of the outer circular cascade is equal to or more than the number of guide vanes of the inner circular cascade. Further, there is a gap between the leading edges of the guide vanes of the outer circular cascade and the trailing edges of the guide vanes of the inner circular cascade. EP-A-O 556 895 shows a vaned diffuser which comprises an inner circular cascade and separated by a slit therefrom an outer circular cascade. The number of guide vanes of the outer circular cascade is equal to the number of guide vanes of the inner circular cascade. DE-A-10 53 714 shows a vaned diffuser which comprises a single circular cascade. SU-A-879 047 shows a vaned diffuser which comprises an inner circular cascade and an outer circular cascade. The number of guide vanes of the outer circular cascade is equal to the number of guide vanes of the inner circular cascade. The trailing edges of the guide vanes of the inner circular cascade are offset from the guide vanes of the outer circular cascade. US-A-4 824 325 discloses a diffuser wherein the number of guide vanes of the inner circular cascade is more than the number of blades of the impeller and is more than the number of guide vanes of the outer circular cascade. In the vaned diffuser, the leading edges of the guide vanes of the outer circular cascade are spaced apart from the trailing edges of the guide vanes of the inner circular cascade in the radial direction. This arrangement is for introducing or guiding the flow into the guide vanes of the outer circular cascade after wake flow by the guide vanes of the inner circular cascade has disappeared. Accordingly, it is considered that if the leading edges of the guide vanes of the outer circular cascade are made close to the trailing edges of the guide vanes of the inner circular cascade in the radial direction, the flow flows into the outer circular cascade before the wake flow due to the guide vanes of the inner circular cascade disappears and therefore the energy loss increases. As described above, a centrifugal compressor provided with a vaned diffuser is generally high in efficiency as compared with the centrifugal compressor which has the vaneless diffuser, but has a disadvantage that the operating range is narrow. A centrifugal compressor which has solved the problem to have a wide operational range has been proposed. However, a problem that the noise is high has still not been solved. It is the object of the invention to provide a centrifugal compressor of the generic kind, which has the characteristics that efficiency is high, operating range is wide and noise is also low. This object is achieved with the centrifugal compressor of the generic kind, in that according to the invention the number of guide vanes of the inner circular cascade of said vaned diffuser is 1.5 times to 1.9 times the number of blades of said impeller and the number of guide vanes of the inner circular cascade is more than the number of guide vanes of the outer circular cascade. Preferably each of the guide vanes of the inner circular cascade of the vaned diffuser has a solidity, i.e. vane length/cascade mean pitch, which is equal to or less than 1. It is convenient that the leading edge of each of the guide vanes of the outer circular cascade of said vaned diffuser is different in position in a circumferential direction from the trailing edge of an associated one of the guide vanes of the inner circular cascade. According to the invention, non-uniformity of the flow distribution in the pitch direction at the outlet of the impeller of the centrifugal compressor is reduced and, accordingly, a fluctuating component of the flow entering into the vaned diffuser is reduced. Accordingly, the compressor noises are considerably reduced. Moreover, there is an advantage that, since frictional loss of the diffuser is reduced, efficiency of the compressor is also improved. Embodiments of the invention are further explained referring to drawings in which Fig. 1 is a sectional view taken along line VII-VII in Fig. 2, showing a first example of the centrifugal compressor according to the invention; Fig. 2 is a longitudinal cross-sectional view, showing the first example of the centrifugal compressor; Fig. 3 is a sectional view similar to Fig. 1, showing a second example of the centrifugal compressor according to the invention; Fig. 4 is a sectional view similar to Fig. 1, showing a third example of the centrifugal compressor according to the invention and Fig. 5 is a sectional view similar to Fig. 1, showing a fourth example of the centrifugal compressor according to the invention. Fig. 1 and 2 show a centrifugal compressor comprising an impeller 2 with blades 13, a vaned diffuser 7, a scroll casing 8 downstream of the diffuser 7 and a suction pipe 9 upstream of the impeller 2. The vaned diffuser 7 comprises a pair of diffuser plates 3 and 4, and two kinds of guide vanes 11 and 12 which are arranged between the diffuser plates 3 and 4 in the form of double circular cascades as shown. The number of short guide vanes 11 at an inner circular cascade is twice the number of long guide vane 12 at an outer circular cascade, and is approximately 1.8 times the number of blades 13 of the impeller 2. Further, the solidity (vane length/cascade mean pitch) of the guide vanes 11 and 12 is equal to or less than 1. The guide vanes 12 are arranged so that their respective leading edges are positioned on a circle of a radius r c while the guide vanes 11 are arranged so that their respective trailing edges are positioned on a circle of a radius r b . The radius r c is slightly larger than the radius r b . The leading edges of the guide vanes 12 are arranged so as to offset peripherally toward pressure surfaces 11a of the guide vanes 11 from extensions in a lengthwise direction of the guide vanes 11. With this arrangement, when the centrifugal compressor runs, flow distribution at an outlet of an impeller 2 is uniformized in a vane pitch direction, and fluctuation in flow is reduced, since the number of the guide vanes 11 at an inner circular cascade are more than the blades 13 of the impeller 2. Thus, the blade passage frequency component of the the noise is reduced. Further, since the frictional loss in the latter half of the flow passage between the vanes is reduced, there can be produced high efficiency. Since a throat 10 is formed only partially in the flow passage between vanes, surqe and choke do not occur easily, and it is possible to secure the wide operating range. Since development of boundary layers on suction surfaces 12b of the guide vanes 12 on the outer circular cascade is restrained respectively by jet which are blown from narrow flow passages formed or defined between pressure surfaces lla of the guide vanes 11 at the inner circular cascade and the suction surfaces 12b of the guide vanes 12 of the second row, performance is improved. In this connection, a gap in the radial direction between the two cascades of the guide vanes 11 and 12 is reduced. Thus, the compressor size is reduced. Fig. 3 shows a vaned diffuser 7 which comprises a pair of diffuser plates 3 and 4 and two kinds of guide vanes 11 and 12 which are arranged between the diffuser plates 3 and 4 in the form of double circular cascades as shown. The number of short guide vane 11 at an inner circular cascade is 3 times the number of long guide vanes 12 at an outer circular cascade, and is approximately 1.8 times the number of blades 13 of an impeller 2. The solidity of the two cascades of guide vanes 11 and 12 are equal to or less than 1. The guide vanes 12 are arranged so that their respective leading edges are positioned on a circle of a radius r c while the guide vanes 11 are arranged so that their respective trailing edges are positioned on a circle of a radius r b . The radius r c is slightly larger than the radius r b . The leading edges of the guide vanes 12 are arranged so as to offset in a peripheral direction toward pressure surfaces lla of the guide vanes 11 from extensions in a lengthwise direction of the guide vanes 11. With this arrangement, when the centrifugal compressor runs, distribution of flow at the outlet of the impeller 2 in the vane pitch direction is uniformized, and fluctuation in flow is reduced since the number of guide vanes 11 on the inner circular cascade is more than the blades 13 of the impeller 2. Accordingly, the noise of the compressor is reduced. Further, since the frictional loss of the latter half of the flow passage between the vanes is reduced, there can be produced high efficiency. Since a throat 10 is formed partially only in a flow passage between the vanes, choke and surge do not occur easily so that a wide operating range is secured. Since a gap in the radial direction between the two cascades of guide vanes 11 and 12 is reduced, it is possible to reduce the compressor size. Development of the boundary layer on a suction surface 12b of the guide vane 12 is restrained by jet which is blown off from the narrow flow passage which is formed between the pressure surface 11a of the guide vane 11 and the suction surface 12b of the guide vane 12. Thus, the diffuser performance, i.e., the compressor performance is improved. Fig. 4 shows a vaned diffuser 7 comprising a pair of diffuser plates 3 and 4 and two kinds of guide vanes 11 and 12 which are arranged between the diffuser plates 3 and 4 in the form of double circular cascades as shown. The number of short guide vanes 11 on an inner circular cascade is 1.5 times the number of long guide vanes 12 on an outer circular cascade, and is of the order of approximately 1.8 times the number of blades 13 on the impeller 2. The solidities of the respective guide vanes 11 and 12 on the two cascades are equal to or less than 1. The guide vanes 12 are arranged so that their respective leading edges are positioned on a circle of a radius r c while the guide vanes 11 are arranged so that their respective trailing edges are positioned on a circle of a radius r b . The radius r c is slightly larger than the radius r b . A half of the guide vane 12 on the outer circular cascade have respective leading edges thereof which are so arranged as to be positioned on extensions in a lengthwise direction of the guide vanes 11. With this arrangement, when the centrifugal compressor runs, flow from an outlet of the impeller 2 is uniformized in a vane pitch direction, since the number of guide vanes 11 on the inner circular cascade is more than the blades 13 of the impeller 2. Thus, fluctuation in flow is reduced. Accordingly, the noise of the compressor is reduced. Since the frictional loss of the latter half of the flow passage between the vanes of the diffuser 7 is reduced, there can be produced high efficiency. Since a throat 10 is formed partially in the flow passage between the vanes of two cascades of guide vanes 11 and 12, it is possible to secure a wide operating range. Development of the boundary layers of suction surfaces 12b of the guide vanes on the outer circular cascade is restrained by jet which is blown from a narrow flow passage formed between suctionsurface 12b of the guide vanes 12 and the pressure surface lla of the guide vanes 11. Accordingly, performance is further improved. Fig. 5 shows a vaned diffuser, the radial lengths of the guide vanes 11 and 12 of which are different from those in Fig. 4. The vaned diffuser 7 of the centrifugal compressor comprises a pair of diffuser plates 3 and 4 and two kinds of guide vanes 11 and 12 which are arranged between the diffuser plates 3 and 4 in the form of double circular cascades as shown. The guide vanes 11 provided at an inner circular cascade are sliahtlv longer than the guide vanes 12 provided at an outer circular cascade. The number of slightly longer guide vanes 11 on the inner circular cascade is twice the number of slightly short guide vanes 12 on the outer circular cascade, and is approximately 1.8 times the number of blades 13 on the impeller 2. Further, the solidity of the guide vanes 11 of the inner circular cascade is greater or larger than 1, and a throat 10 is formed at all the flow passage between vanes 11. The solidity of the guide vane 12 of the outer circular cascade is equal to or less than 1, and a throat 10 is not formed in the flow passage between vanes 12. The leading edges of the guide vanes 12 of the outer circular cascade are so arranged as to offset peripherally toward pressure surfaces 12a from extensions of the guide vanes 11. With this arrangement, when the centrifugal compressor runs, the flow at the outlet of the impeller 2 is uniformized in the vane pitch direction, and fluctuation in flow is reduced, since the number of guide vanes 11 on the inner circular cascade of the diffuser 7 is more than the blades 13 of the impeller 2. Thus, the noise of the compressor is reduced. Since the frictional loss of the latter half of the flow passage between the vanes is reduced, there can be produced performance high in efficiency. Since the throats 10 are formed in all of the flow passages between vanes of the guide vanes 11 on the inner circular cascade, surge and choke are apt to occur as compared with the embodiments illustrated in Fig. 1 to Fig. 4. Thus, there is possibility that the operating range is slightly narrowed. The embodiments illustrated in Fig. 1 to Fig. 5 are arranged such that the total number of guide vanes 11 and 12 of the diffuser 7 is approximately of the order of 1.5 times or 1.8 times the number of blades 13 of the impeller 2. However, it is possible that these numbers of vanes are within a range of from 1.5 times to 1.9 times. However, if this is equal to or less than 1.5 times, function of uniformizing flow distribution at the outlet of the impeller 2 in the vane pitch direction is reduced, and the blade passage frequency component which is dominant to the noise of the compressor cannot be reduced. Moreover, if it is larger than the order of 1.9 times, the efficiency falls in view of the reasons that frictional loss increases because the number of vanes of the diffuser 7 increases, and that deceleration is reduced because the vane length is short.

Claims: A centrifugal compressor comprising a centrifugal impeller (2) and a vaned diffuser (7) arrangend downstream of said impeller (2), said vaned diffuser (7) including a pair of diffuser plates (3, 4) and a plurality of guide vanes (11, 12) arranged between said pair of diffuser plates (3, 4) in a circular cascade manner, said circular cascade of said vaned diffuser (7) comprising an inner circular cascade and an outer circular cascade, wherein the leading-edge radius (r c ) of the guide vanes (12) of the outer circular cascade is substantially the same in level as the trailing-edge radius (r b ) of the guide vanes (11) of the inner circular cascade, characterized in that the number of guide vanes (11) of the inner circular cascade is 1.5 times to 1.9 times the number of blades (13) of said impeller (2) and in that the number of guide vanes (11) of the inner circular cascade is more than the number of guide vanes (12) of the outer circular cascade. A centrifugal compressor according to claim 1, characterized in that each of the guide vanes (11) of the inner circular cascade of the vaned diffuser (7) has a solidity, i.e. vane length/cascade mean pitch, which is equal to or less than 1. A centrifugal compressor according to claim 1, characterized in that the leading edge of each of the guide vanes (12) of the outer circular cascade of said vaned diffuser (7) is different in position in a circumferential direction from the trailing edge of an associated one of the guide vanes (11) of the inner circular cascade.

Inventors: KOBAYASHI HIROMI, MIURA HARUO, NISHIDA HIDEO, TANAKA SADASHI, YOSHIKAI HIROTO, KOBAYASHI, HIROMI, MIURA, HARUO, NISHIDA, HIDEO, TANAKA, SADASHI, YOSHIKAI, HIROTO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1994
Main classification: F04D29
IPC codes: F04D29